This isn't a mule, it's a regular issue, though fairly rare. During this period, they also struck VRBS and Constantinopolis obverses with GLORIA EXERCITVS reverses. Though your mintmark is not clear, it is probably from Heraclea (something like SMHA or •SMHA) -- see RIC VIII Heraclea 49/ 56. struck A.D. 347- 348